South Korea and the US may raise their state of readiness against the military threat posed by North Korea to the second-highest level in response to the sinking of the patrol vessel Cheonan.

Level one, the most serious condition of the five-level scale known as WatchCon, has not been in force since the end of the Korean war in 1953. Level two was last reached when North Korea conducted a nuclear test in 2009.

The deliberations come as South Korea's president, Lee Myung-bak, is expected to refer the sinking in March of the Cheonan to the UN security council.

The United Nations command, which signed the armistice ending the war on the south's behalf, yesterday launched its own investigation into whether the sinking of the South Korean ship, which led to the death of 46 sailors, violated the truce.

South Korea's defence ministry is understood to have contacted its counterpart in Pyongyang to ask it to attend a meeting with the UN command to discuss the findings. These moves follow the publication on Thursday of a multinational investigation into the sinking which concluded that a North Korean submarine sank the ship. A motive for the attack remains unclear.

A raising of the WatchCon level would trigger increased US-South Korean reconnaissance and intelligence-gathering efforts targeting the north. Also under consideration is an additional joint US-South Korean military exercise, perhaps as early as June, to signal to North Korea Washington's support for Seoul.

The US secretary of state, Hillary Clinton, appears to be facing an uphill diplomatic struggle to win China's support for penalising North Korea over the sinking of the warship.

China, North Korea's primary ally and financial supporter, has been neutral on the conclusions of the report that found Pyongyang responsible for firing the torpedo. China's president, Hu Jintao, will travel to South Korea this week for a three-nation summit, which will also include Japan, on the South Korean island of Jeju where the attack is expected to be a subject of further debate.

• This article was amended on Sunday 30 May 2010. China's president is not Wen Jiabao. Hu Jintao is president; Wen Jiabao is premier. This has been corrected.